1. Field of the Invention
The present invention relates to an image recording method in which multicolor, multi-gradation expression is allowed by varying dot lengths.
2. Description of the Related Art
As a means of obtaining a multicolor, multi-gradation recording, a dot gradation expression, for example, can be suggested in which the respective dot areas are varied in accordance with gradations so as to express desired gradations. In this case, in order to correspond to the multicolor recording, the dots of respective colors having the same shape are superposed and recorded. However, deviation in registration may occur every recording and thereby generating a color moire.
The above-described color moire can be prevented by improving the precision of machinery. However, it is technologically difficult to improve the precision. Therefore, in the field of offset printing or the like, the color moire has been prevented by changing the angles of dots of respective colors. Namely, instead of arranging the dots grid-like and regularly, the angles at which the dots are arranged (dot arrangement angles) are changed to, for example, 15.degree., 30.degree., 45.degree., or 75.degree. with respect to the main scanning direction.
Alternatively, a so-called FM screening, in which dots of a predetermined size are randomly arranged in accordance with gradations, are used so that the color moire is suppressed by random dots of the respective colors which do not have directions.
The above-described respective means can effectively suppress the occurrence of color moire in a case in which the resolution in the main scanning direction and the resolution in the sub-scanning direction are set smaller than the unit of a pixel. However, when the resolution in the main scanning direction is a line head such as a thermal head, a liquid crystal head, an LED (light emitting diode) head, or an EL (electroluminescence) head, which are specified by the size of a recording element and are substantially set to the unit of a pixel, the degrees of freedom in changing the arrangement of dots is limited to the sub-scanning direction. Accordingly, the moire stripes cannot be effectively suppressed.
In order to solve the above circumstances, a color image recording method which is disclosed in Japanese Patent Application Laid-Open No. 5-155057 has been proposed. Namely, in order to prevent the respective pixels of three primary colors from being arranged in the main scanning direction, the pixels are shifted a substantially half pitch in the sub-scanning direction. At the same time, the pixel density of one of the three primary colors is halved in the sub-scanning direction. Consequently, even if the deviation in registration occurs, the color pattern of the adjacent two columns are maintained in a complementary chromatic relationship. Accordingly, overall tone does not change, and irregularities in color are not conspicuous.
As described above, various means have been proposed to prevent color moire when a multicolor, multi-gradation recording is effected.
However, even if the multicolor, multi-gradation expression is effected by changing the dot arrangement angles, the occurrence of color moire called a Rosetta pattern, which causes graininess, cannot be prevented.
Moreover, according to the color image recording method disclosed in Japanese Patent Application Laid-Open No. 5-155057, in the case of three colors, the color moire can be effectively suppressed even if the resolution in the main scanning direction is limited. However, the color moire cannot be prevented in the case of multicolor, multi-gradation recording by a four-color separation, which is widely used in printing.